The office environment has, for many years, been the home of objectionable noise generators, namely, typewriters and high speed impact printers. Where several such devices are placed together in a single room, the cumulative noise pollution may even be hazardous to the health and well being of its occupants. The situation is well recognized and has been addressed by the technical community as well as by governmental bodies. Attempts have been made to reduce the noise by several methods: enclosing impact printers in sound attenuating covers; designing impact printers in which the impact noise is reduced; and designing quieter printers based on non-impact technologies such as ink jet and thermal transfer. Also, legislative and regulatory bodies have set standards for maximum acceptable noise levels in office environments.
Loudness levels measured on a dBA scale represent human perceived levels of loudness as opposed to absolute values of sound intensity. When considering sound energy represented in dBA (or dB) units, it should be borne in mind that the scale is logarithmic and that a 10 dB difference means a factor of 10, a 20 dB difference means a factor of 100, 30 dB a factor of 1000 and so on. Typically, conventional impact printers generate an average noise in the range of 70 to just over 80 dBA, which is deemed to be intrusive. When reduced to the 60-70 dBA range, the noise is construed to be objectionable. Further reduction of the impact noise level to the 50-60 dBA range would improve the designation to annoying. Clearly, it would be desirable to reduce the impact noise to a dBA value in the low to mid-40's, a very aggressive dropoff in printer impact noise.
The printing noise referenced above is of an impulse character and is primarily produced as the hammer impacts and drives the type character pad against the ribbon, the print sheet and the platen with sufficient force to release the ink from the ribbon. The discussion herein will be directed solely to the impact noise which masks other noises in the system. However, once the impact noise has been substantially reduced, the other noises will no longer be extraneous. Thus, the design of a truly quiet printer requires the designer to address reducing all other noise sources, such as those arising from carriage motion, character selection, ribbon lift and advance, as well as from miscellaneous clutches, solenoids, motors and switches.
Since it is the impact noise which is modified in the present invention, it is necessary to understand the origin of the impact noise in conventional ballistic hammer impact printers. In a typical daisywheel printer, a hammer mass of about 2.5 grams is driven ballistically by a solenoid-actuated clapper; the hammer hits the rear surface of the character pad and impacts it against the ribbon/paper/platen combination, from which it rebounds to its home position where it must be stopped, usually by another impact. This series of impacts is the main source of the objectionable noise.
In conventional printers, the total dwell time of the platen deformation impact, i.e. the hammer against the ribbon/paper/platen combination, is typically in the vicinity of 100 microseconds. Yet, at a printing speed of 30 characters per second, the mean time available between character impacts is about 30 milliseconds. Clearly, there is ample opportunity to significantly stretch the impact dwell time to a substantially larger fraction of the printing cycle than is typical of conventional printers. For instance, if the dwell time were stretched from 100 microseconds to 6 to 10 milliseconds, this would represent a sixty- to one hundred-fold increase, or stretch in pulse width relative to the conventional. By extending the deforming of the platen over a longer period of time, an attendant reduction in noise output can be achieved as is fully explained in the "Theory of Operation" section of the copending application assigned to the same assignee as the present case and bearing U.S. Ser. No. 751,169 filed July 2, 1985 in the name of Andrew Gabor and entitled "Quiet Impact Printer." The copending application is incorporated herein by reference.
The operator is provided with a number of ribbon materials for use in today's printers. Each ribbon type is formulated for a particular purpose and has its own ink release characteristics, but all are designed for use in the conventional high force, low mass, short dwell time printers. Conceivably, a printer manufacturer can manufacture and designate ribbon types and formulations having release characteristics tailored to be particularly compatible with his printer. However, it would be desirable if the presently commercially available ribbon types could be used in the low mass, long dwell time printer of the present invention and exhibit comparable or improved print quality. The real challenge in printer design is its ability to obtain high print quality on different types of multi-part forms. Whereas the printer manufacturer may designate particular ribbons to be used with his printer, the customer chooses his multi-part forms independently of the printer.
A brief discussion of the conventional ribbon types and multi-part forms, and their characteristics will aid in an understanding of the improved release mechanisms to be described below. "Single strike" ribbons comprise a layer of a dry waxy ink substance on a polyethylene substrate. When struck by a print element, propelled by a hammer, the wax fractures in the character configuration, is broken away from its substrate and is completely transferred to and adheres to the image receptor sheet. This process actually comprises fracturing and pushing out a character shaped plug. Multi-part forms with interleaved carbons have the same ink release mechanism since the carbon sheets comprise a similar waxy substance coated on a low grade paper. On the other hand, multi-part forms with encapsulated inks mark the paper sheet when the capsules are broken and their liquid contents react with the coating on the sheet to form a colored image.
"Multi-strike" ribbons may be reviewed as micro-stamp pads, or a thin porous sponge filled with ink, supported upon a polyester substrate, such as that commonly known as Mylar. This substrate material is strong enough to withstand plural impacts on very closely overlapping areas. "Fabric" ribbons are similar to multi-strike but comprise a cloth substrate whose fibers are impregnated with a liquid ink.
"Correctable" inks are characterized by being able to be lifted off the paper shortly after printing. The ink is carbon (or other colorant) in a friable plastic on a polyethylene substrate. A description of this type of ribbon is to be found in U.S. Pat. No. 3,825,470 entitled "Adhesively Eradicable Transfer Medium" (D. L. Elbert et al). The coating is easily fractured on impact and is pressure sintered into a cohesive character shaped mass on the paper image receptor. Since the mass is a plastic, it doesn't penetrate deeply into the paper fibers. Thus, during correcting, an adhesive tape is used to attach to the cohesive mass and pull it off the paper. Given enough time, about five minutes, the plastic particles will eventually flow into the paper to form a permanent image.
It has been determined that the single strike ribbon and multi-part carbon forms will have improved release if a shear component, i.e. tangent to the platen, is added to the impact force. This component will induce a "peeling" action as the substrate is shifted by the character element while the ink "plug" is held stationary on the paper. Clearly, the shear component will have absolutely no advantageous effect on the ink release of the multistrike or fabric ribbons which are in effect micro-sponges. Pressure alone will squeeze the liquid ink out of the pores, the direction of the pressure is of little or no consequences. Similarly the correctible ribbon does not respond to shear.
Whereas, the multi-strike, fabric and correctable ribbons will not respond to shear forces, they will respond favorably to an extended application of pressure. It is well known that the longer one applies pressure to a sponge the more fluid will be released. On the other hand, the ink release of single strike ribbons and multi-part interleaved carbon forms will not be improved by the extended dwell. Once sufficient force has been applied to fracture the solid ink coating it is transferred to the paper. Further application of force will not fracture it further. Therefore, extended dwell is irrelevant to these marking materials.
It is the primary object of the present invention to provide a printer which will operate several orders of magnitude quieter than printers typical in today's marketplace, while obtaining the same print quality, regardless of the type of ribbon or multi-part form used therein.